1. Field of the Invention
The present invention relates generally to a system and method for compensating timing errors in an OFDM/CDMA communication system, and in particular, to a system and method for continuously compensating timing errors by detecting a pilot signal inserted in a symbol unit and using a phase difference line.
2. Description of the Related Art
In general, an OFDM/CDMA (Orthogonal Frequency Division Multiplexing/Code Division Multiple Access) communication system uses multiple carriers having orthogonality. In the OFDM/CDMA communication system, it is very important to maintain the orthogonality among the multiple carriers during demodulation, since maintaining the orthogonality among the multiple carriers at the receiver is closely related to the call quality. A receiver in the OFDM/CDMA communication system also performs frame sync (synchronization), sampling sync and carrier frequency sync in order to demodulate an OFDM signal transmitted from a transmitter, similar to receivers in other mobile communication systems. Since the OFDM/CDMA communication system must maintain the orthogonality during demodulation by using multiple carriers, it is necessary to perform accurate synchronization.
FIG. 1 illustrates a block diagram of a general OFDM/CDMA communication system, and FIG. 2 illustrates a general method for inserting pilot signals. A description of FIGS. 1 and 2 follows below.
The structure of a transmitter in a general OFDM/CDMA communication system is illustrated in FIG. 1. A pilot sample inserter 101 generally receives a data symbol comprised of N spread data samples and inserts a pilot sample at regular intervals as shown in FIG. 2. The pilot sample inserting method is divided into (1) an inserting method for delaying actual sample data in a position where the pilot sample is to be inserted; (2) a puncturing method for inserting-after-puncturing the actual sample data (i.e., puncturing a specific bit and then inserting the actual sample data in the bit-punctured position). In the description hereinbelow, the puncturing method is used for pilot sample insertion. The data symbol is a signal spread with a code having a rate of N times. A serial/parallel (S/P) converter 103 separates the pilot symbol output from the pilot sample inserter 101 into N data samples, and provides the separated data samples in parallel to an inverse fast Fourier transform (IFFT) block 105. The IFFT 105 performs inverse fast Fourier transform, i.e., OFDM modulation on the N data samples output from the S/P converter 103, and outputs the N OFDM-modulated OFDM data samples in parallel. A parallel/serial (P/S) converter 106 receives in parallel the OFDM data samples output from the IFFT 105, and outputs an OFDM symbol comprised of N samples to a guard interval inserter 107. The guard interval inserter 107 then inserts, at the head of the OFDM symbol, a guard interval determined by copying the last G data samples (hereinafter, referred to as “copied data samples”) out of the N OFDM data samples. A digital-to-analog converter (DAC) 109 converts the OFDM symbol output from the guard interval inserter 107 to an analog OFDM signal and transmits the converted analog OFDM signal.
The OFDM signal transmitted by the transmitter is received by an analog-to-digital converter (ADC) 111 of a receiver. The ADC 111 converts the received OFDM signal to a digital OFDM symbol comprised of a guard interval and N OFDM data samples and provides the converted OFDM symbol to a guard interval remover 112. The guard interval remover 112 removes the guard interval included in the provided OFDM symbol, and outputs a pure OFDM symbol comprised of N OFDM data samples. The ADC 111 and the guard interval remover 112 operate according to a prescribed timing error estimation signal. An S/P converter 113 separates the OFDM symbol output from the guard interval remover 112 into N OFDM data samples, and outputs the N OFDM data samples in parallel. A fast Fourier transform (FFT) block 114 performs fast Fourier transform, i.e., OFDM demodulation on the N data samples received in parallel from the S/P converter 113, and outputs N OFDM-demodulated data samples. The N data samples are converted to a serial data symbol by a P/S converter 115 and then provided to a pilot sample detector 116. The pilot sample detector 116 detects pilot data samples inserted in the data symbol output from the P/S converter 115, and provides the detected pilot data samples to a timing compensator 117 and the data samples to a despreader 119. Receiving the pilot data samples from the pilot sample detector 116, the timing compensator 117 calculates a timing error using the FFT property shown in Equation (1) below, compensates the calculated timing error, and outputs a timing error estimation signal to the ADC 111 and the guard interval remover 112.
                                          x            ⁡                          [                              n                -                                  n                  0                                            ]                                ⇔                                    X              ⁡                              (                k                )                                      ⁢                          W              N                              k                                  n                  0                                                                    ,                              where            ⁢                                                  ⁢                          W              N                                =                      ⅇ                                          -                j                            ⁢                                                          ⁢                                                2                  ⁢                  π                                N                                                                        (        1        )            
In Equation (1), x[n−n0] indicates a transmission signal which is time-delayed by n0, and X(k)WNkn0 indicates a received signal which is linear phase shifted by WNkn0 according to the delay time n0.
A detailed operation of the timing compensator 117 will be described in detail with reference to Equation (1). The timing compensator 117 calculates a difference between a phase of the pilot sample detected by the pilot sample detector 116 and a previously known reference phase, and estimates a timing error using a fluctuation of the calculated difference value. The despreader 119 despreads the data symbol received from the pilot sample detector 116.
As described above, the OFDM/CDMA communication system has two types of timing compensation methods.
The first method is to insert a pilot data sample between original data samples in a specific period or pattern. In this case, the OFDM/CDMA communication system processes the data in a symbol unit at the receiver, since the respective samples in one symbol have the same information. However, when this method is used, the data is shifted back by the number of the pilot samples, so that transmission is not performed in the symbol unit. Further, the position of the sample where the actual data symbol starts is continuously changed, so that the receiver must continuously search the start position of the actual data symbol.
The second method is to puncture some of the actual data samples in a specific period or pattern and insert a pilot sample in the punctured data sample position. In this case, significant noise is generated because the sample data, which is the original data, is punctured when the receiver despreads the actual sample data.
Further, in the receiver, a frequency error in a time domain is expressed by timing changing in a frequency domain after passing the FFT stage. If the frequency error larger than a sub-carrier band passes the FFT stage, one or more samples are shifted, so that another data sample is located in a position where the pilot data sample is to be located. This is because the positions of the pilot data samples in the symbol are not continuous. In this case, it is not possible to obtain required information. Thus, it is not possible to compensate for the timing error in the conventional method.
More specifically, in an ideal system, a phase difference between the received pilot data sample and the reference data sample is (2πnek)/N and has a linear property with respect to an index ‘k’, as shown in Equation (1). That is, it is possible to calculate a timing error ne by calculating a slope for the index ‘k’ of the phase difference and then dividing the calculated slope by 2π/N. However, due to the phase characteristic in which the value is limited to ±π, it is not possible to obtain a linear phase difference line and the phase difference line has an abrupt fluctuation of about ±2πat around ±π. In this case, a process for converting the phase difference line to a linear phase difference line is required. This raises a more serous problem in a non-ideal system. A factor affecting the phase difference line includes a frequency error, a common phase error (CPE), noises, and non-cyclic shift.
In the receiver, a frequency error ke can be divided into a frequency error kei of a multiple of one-data sample interval and a frequency error ked of within one-data sample interval. The frequency error ke in the time domain is expressed in timing changing in the frequency domain after passing the FFT stage, and if a frequency error kei occurred longer than a one-sample period passes the FFT stage, the respective pilot data samples in the data symbol are shifted by over one data sample, so that a data sample other than the original pilot data sample is received, thus making it difficult to calculate an accurate phase difference. In addition, the frequency error ked also affects the phase difference line caused by fluctuation of the phase. In this case, the phase difference line is formed as shown in FIG. 3. In this phase difference line, the dots denote the pilot data samples.
Therefore, in order to use the conventional timing error compensation method, the OFDM/CDMA communication system should necessarily compensate the frequency error of over the sub-carrier band before timing estimation.
The number of pilot data samples is also an important factor affecting the performance. As the timing error increases more and more, the fluctuation of the phase increases and the number of transitions also increases, so that many pilot data samples are required. For example, one data symbol requires the pilot samples over four times of the timing error.